Compounds containing nitrogen including but not limited to nitrates, nitrites, nitro-organic compounds, ammonia, amines, and amides are often present together in various combinations in non-radioactive aqueous mixed waste streams such as sewage, sewage sludge, nitrate or nitrite wastes at metal finishing plants, and chemical and munitions plants. Nuclear materials production facilities also generate waste streams containing both nitrogen bearing compounds and radioactive materials.
In many waste or process streams, the concentration of nitrogen compounds is below 1% which is insufficient for cost effective removal of nitrogen compounds by traditional means. Removal of nitrogen from nitrogen bearing streams of higher concentration may be precluded by the presence of hazardous chemicals and/or radioactivity. Moreover, nitrogen compounds at any concentration in a waste stream, present problems such as nitrous oxide (NO.sub.x) emission upon disposal by incineration, and algae bloom induced eutrophication upon disposal by drainage into bodies of water.
Of the many methods of denitrification, very few are effective for anything other than a single nitrogen containing compound For example, The Nalco Water Handbook, 1979, pp. 6-11, states that "[t]he only chemical process that removes nitrate is anion exchange". However, the anion exchange process suffers from a number of disadvantages including 1) other nitrogen compounds are unaffected by the anion exchange, 2) additional chemicals are required to regenerate the anion exchange resin and 3) additional chemicals are required to regenerate the anion exchange resin and a waste stream is produced upon resin regeneration.
Further examples of single nitrogen compound removal include methods of ammonia removal The Handbook (pp. 6-10) also states that "[a]mmonia can be removed by degasification, by cation exchange on the hydrogen cycle, and by adsorption on certain clays, such as clinoptilolite". The disadvantage of these processes is that since they are primarily directed toward removal of ammonia, other compounds containing nitrogen are generally unaffected. A further disadvantage of these processes is that the pH of the waste stream must be raised to increase the vapor pressure of aqueous ammonia.
Another method of ammonia removal is by addition of chlorine to form nitrogen gas and hydrochloric acid. For purposes of toxic waste remediation, it is undesirable to handle chlorine or produce hydrochloric acid, and not all nitrogen compounds will release nitrogen gas upon addition of chlorine.
Hydrazine (N.sub.2 H.sub.4), may be removed by reaction with dissolved oxygen to produce nitrogen gas and water. However, any other nitrogen compounds that may be present remain unaffected by this reaction.
Each of the denitrification processes described so far are effective for removing one type of nitrogen compound. Removal of multiple nitrogen compounds by these methods requires use of multiple methods.
There are currently two methods capable of removing multiple nitrogen compounds, bacterial processing and incineration. Conventional bacterial systems usually require a settling pond or biological reactor, are carried out at temperatures below 30.degree. C., require equipment to handle great quantities of air and require residence times on the order of days to reduce nitrogen compound concentrations below acceptable limits.
In cases where nitrogen bearing waste streams are incinerated, undesirable nitrous oxide (NO.sub.x) emissions, components of smog, are produced. NO.sub.x can be combined with ammonia and destroyed by gas phase reactions at temperatures between 1000.degree. C. and 1100.degree. C. (known as thermal deNOx) or by selective catalytic reduction, at temperatures between 650.degree. C. and 750.degree. C. in the presence of a catalyst to convert the NO.sub.x to nitrogen, oxygen, and water. Disadvantages of treating nitrous oxides in the gas phase include, but are not limited to, 1) the size of the equipment required for handling gases, 2) the high temperature operation, 3) handling potentially corrosive condensate after the gas stream is cooled and 4) the cost of disposal of a spent catalyst after processing radioactive wastes.
Nitrogen compounds may be converted to a second nitrogen compound, but this does not fully remove nitrogen compounds. For example, cases where waste streams have a high chemical oxygen demand (COD) from the presence of carbonaceous and nitrogenous compounds, wet air oxidation can be used to oxidize most or all of the carbon portion of the waste. J. R. Heimbuch and A. R. Wilhelmi stated in their publication "Wet Air Oxidation--A Treatment Means for Aqueous Hazardous Waste Streams", December, 1985, Journal of Hazardous Materials, page 192: "A significant advantage of wet air oxidation is that there are minimal air pollution problems. Contaminants tend to stay in the aqueous phase. The small amount of gas that is discharged consists mainly of spent air and carbon dioxide. NOx emissions are not observed because nitrogen compounds are converted to ammonia." Thus, while wet air oxidation is effective for destroying the carbonaceous portion of the waste and converting the nitrogenous portion to ammonia, wet air oxidation as currently practiced, does not remove the nitrogen in the ammonia present in the aqueous stream.
In cases of waste streams having a plurality of nitrogen compounds, removal of nitrogen is a difficult and expensive task. Prior to the instant invention, only bacterial action and incineration were capable of removing a plurality of nitrogen compounds from an aqueous waste stream. However, neither of these approaches have nitrogen gas as the predominant nitrogenous end product and both of these approaches suffer from the previously mentioned disadvantages, especially when radioactive waste streams are considered.
The present invention is therefore, directed toward a method of removing a plurality of nitrogen containing compounds from an aqueous waste stream resulting in release of nitrogen as nitrogen gas without formation of nitrous oxides such as NO, NO.sub.2 and N.sub.2 O.sub.4. The method of the present invention relies upon aqueous phase reactions at moderate temperatures and pressures without the use of a catalyst and without the subsequent regeneration and/or disposal of a catalyst in both non-radioactive and radioactive waste treatment.